What does the Aufbau principle state?

It is the principal that has all of the electrons in the lowest possible energy levels.

Lowest energy to highest energy:

1S, 2S, 2P, 3S, 3P, 4S, 3D, 4P...

or in other Words we can say , "According to Aufbau's principal the electrons enter in an ascending order of energy levels ."

There is also a Little trick to remember whole order of electron configuration according to aufbau's law - remember this thing - 1,22,33,43,45,45,64,567,567

ok now all u have to do is just put S P D and F . put S with a number if the number is coming for the very first time . if the number comes second time put P and if number comes third time put D and if fourth time then put F .and then put the maximum electrons possible to fill in a particular sub shell S=2 P=6 D=10 F=14 now u will have something look like this :

1s2 , 2s2 2p6 ,3s2 3p6 , 4s2 3d10 ,4p6 5s2, 4d10 5p6 ,6s2 4f14 ,5d10 6p6 7s2 , 5f14 6d10 7p6 . which is aufbau's law :)